In general, an automatic transmission mounted in a vehicle includes a plurality of planetary gear sets (planetary gear mechanisms), and a plurality of hydraulic frictional coupling elements such as clutches and brakes. These frictional coupling elements are selectively engaged by hydraulic control to change power transmission paths through the planetary gear sets, thereby providing a plurality of forward gears and usually one reverse gear.
In recent years, there is a growing demand for an increase in the number of forward gears to improve the fuel efficiency of an engine and transmission performance. For example, an automatic transmission suggested includes three planetary gear sets and six frictional coupling elements, and provides eight forward gears by combining two of these frictional coupling elements.
In an automatic transmission with this configuration, however, four of the frictional coupling elements are disengaged in each gear stage. The sliding friction between the friction plates of the disengaged frictional coupling elements or the viscous resistance of the lubricant between the friction plates or any other factor increases the drive loss of the automatic transmission as a whole. This may hinder an improvement in the fuel efficiency, which is the advantage in increasing the number of gears.
To overcome such a problem, PATENT DOCUMENT 1 discloses an automatic transmission including four planetary gear sets, and five frictional coupling elements. Three of the frictional coupling elements are selectively engaged to provide eight forward gears.
In this configuration, the number of the frictional coupling elements that are disengaged in each gear stage decreases to two, thereby reducing the above-described drive loss. All the four planetary gear sets are of a single pinion type, which have a simpler configuration than planetary gear sets of a double pinion type, thereby reducing the whole size of the transmission.
The automatic transmission disclosed in PATENT DOCUMENT 1 has the configuration shown in FIG. 20. Specifically, first, second, third, and fourth planetary gear sets PGa, PGb, PGc, and PGd are arranged in this order from the input side (the left of FIG. 20). A first clutch CLa is arranged between the second and third planetary gear sets PGb and PGc. A second clutch CLb and a third clutch CLc are arranged closer to on an outer periphery and an inner periphery, respectively, between the third and fourth planetary gear sets PGc and PGd. A carrier of the first planetary gear set PGa and a ring gear of the fourth planetary gear set PGd are engaged via a power transmission member x. A ring gear of the second planetary gear set PGb and a sun gear of the third planetary gear set PGc are engaged via a power transmission member y. A carrier of the third planetary gear set PGc and a carrier of the fourth planetary gear set PGd are engaged via a power transmission member z.
With this configuration, the power transmission members x and y cover the outer periphery of the first clutch CLa, and the power transmission members x and z cover the outer peripheries of the second and third clutches CLb and CLc. As a result, each of the clutches CLa, CLb, and CLc is located in a closed space surrounded by the planetary gear sets on the both sides of the clutch, and the power transmission members on the outer periphery of the clutch.